52 2 PATTERNS AND PROBLEMS OF DEVELOPMENT 



from twelve 1/16 blastomeres in various combinations, and even after re- 

 moval of the whole embryonic area up to a certain stage preceding gastru- 

 lation.'" Removal by localized puncture or cautery of parts of the embry- 

 onic shield after its formation indicates, in general, progressive stabiliza- 

 tion of course of development; but apparently normal, or only quantita- 

 tively defective, development is possible after removal of small amounts, 

 even of axial material. Injury of the posterior end of the embryonic region 

 in earlier stages may completely inhibit embryo formation, perhaps by 

 obliteration of the polar gradient. In later stages it results in absence of 

 posterior parts still to be formed at the stage of injury. Recent experi- 

 ments, using the method of implantation of isolated parts of the blasto- 

 derm into the yolk sacs of other embryos, show similar development of all 

 sectors of the blastoderm in pregastrula stages (Mangold, 193 it; W. Luth- 

 er, 1936a, ^). Sectors including the marginal region, when thus implanted, 

 are capable of developing neural tube, notochord, muscle segments, in- 

 testine, auditory vesicles, eye lenses, blood vessels, blood corpuscles, kid- 

 ney canals, and liver tissue. Reconstitution of the extraembryonic sectors 

 apparently represents an approach to development of a new embryonic 

 primordium. Further experiments on removals and transplantations of 

 parts of the blastoderm of the trout also show high capacity for reconstitu- 

 tion in earlier stages (W. Luther, 1937a, b). Combination of two extra- 

 embryonic halves in the blastula stage by replacing the embryonic half by 

 an extraembryonic half results in reconstitution of complete, or almost 

 complete, embryos in 90 per cent of the cases. In early gastrula stages an 

 embryo develops after removal of a lateral half of the embryonic sector, 

 either by movement into its place of material still farther lateral or when 

 it is replaced by a piece from the extraembryonic half. Reconstitution fol- 

 lows removal of either lateral or median part of the embryonic region. In 

 the early gastrula the region capable of development extends about 45° 

 right and left of the longitudinal embryonic axis, but reconstitution some- 

 times occurs after replacement of somewhat more than 90° of the em- 

 bryonic sector by an extraembryonic piece. With progress of gastrulation 

 the reconstitutional capacity of extraembryonic regions of the blastoderm 

 gradually disappears, first in the median region opposite the embryonic 

 sector and from this region progressively on both sides. In the later gas- 

 trula capacity for development corresponds approximately to the pros- 

 pective organ regions. 



Luther's interpretation of these data is that capacity for development 



"Morgan, 1895a; Kopsch, 1896, 1904; Sumner, 1903; W. H. Lewis, 1912a, b; Hoadley, 

 1928. 



